Ceramic matrix composites (CMCs) are being developed for applications up to 1755 K (2700 F). With traditional CMC's, the silicon carbide (SiC) fiber tows in preforms are coated with a boron nitride (BN) coating overlaid with a SiC layer deposited by chemical vapor infiltration (CVI) traditionally termed as “CVI SiC” in the literature. In preforms with CVI SiC partially filling the space (termed as “partial CVI SiC”) between neighboring 0° and 90° BN-coated SiC tows, the remaining space in traditional CMCs used in 1623 K (2462 F) applications is filled by a “filler matrix” consisting of either Silicon (Si), SiC, or both, using a Si melt infiltration (MI) process, CVI, slurry infiltration, polymer impregnation and pyrolysis (PIP) and/or both.
Ceramic matrix composites containing free Si within the CVI SiC and in the filler matrices cannot be used at temperatures above 1623 K (2462 F) due to poor creep properties. Alternatively, the void space between the fiber tows can be filled solely with SiC either by CVI (termed as “full CVI SiC”), which results in good creep properties but with poor crack resistance capabilities or in combination with PIP, slurry infiltration and MI.
Therefore, if a crack propagates either from the protective environmental barrier coating (EBC), the bond coat or internally from a fiber tow, the crack is likely to propagate rapidly through the brittle SiC filler matrix in the CMC leading to considerable matrix damage and corresponding overloading of the fibers and a reduction in composite life. Furthermore, when these cracks are connected to the external surface, oxygen ingress into the interior of the CMC is likely to result in the oxidation of the BN coatings, resulting in a decrease in the CMC life.
Accordingly, it would be beneficial to improve the durability of the matrix and eliminate free Si to allow the SiC/SiC CMCs to be used at temperatures up to 1755 K (2700 F).